Thread cutting devices for sewing machines are known in the art. A conventional thread cutting device for the sewing machine which is mounted downwardly of a throat or a needle plate is shown in FIG. 1.
A stationary blade 1 is firmly held in position downwardly of the throat plate and includes at its tip end a knife or cutting edge 2. The cutting edge 2 extends to the vicinity of a needle guide 3 with an opening through which a needle (not shown) passes. A movable blade 4 is connected to a certain drive means or mechanism (not shown) and is so driven about a shaft 5 in the direction of an arrow as to extend across the stationary blade 1. A cutout 7 is formed in the movable blade 4 on the back side thereof as viewed from FIG. 1 to face the stationary blade 1. The cutout 7 is provided at its corner with a circular hole 6 and serves to guide threads (not shown) extending from needle guide 3 along a path. The circular hole 6 is ground and faced to obtain a cutting edge 8.
The conventional thread cutter is structured as aforementioned and reference will be made to the operation thereof shown in FIGS. 2A, 2B, 3A, and 3B.
FIG. 2A shows the thread cutting device in the stand-by position. In this position, the threads 20 (needle and bobbin threads in this instance) are extended from the needle guide 3 to a workpiece.
FIG. 2B shows the thread cutting device in an operative position where the movable blade 4 starts rotating about shaft 5. The movable blade 4 passes over the needle guide 3 to catch the threads 20 and continues to move with the threads 20 in the direction of the stationary blade 1.
FIG. 3A shows the moving blade positioned to extend over the needle guide 3 and the stationary blade 1. The threads 20 are guided along a path for the movable blade 4 by the cutout 7 formed therein.
FIG. 3B shows the threads when cut. The threads 20 run in the cutout 7 are cut in the optimum direction and at the optimum angle by the cutting edges 2, 8 of the stationary and moving blades 1, 4.
The prior art thread cutting device has, however, disadvantages which are described hereinafter with reference to FIGS. 4A and 4B. Threads which are of a diameter larger than the depth of the cutout 7 may occasionally fail to run into the circular ground hole in the cutting edge 8. This occurrence will inadvertently move up the movable blade 4 to thus separate from the stationary blade. FIG. 4A shows the moving blade 4 located to extend over the needle guide 3 and the stationary blade 1. At this moment, the thicker threads 20 are held or clamped between the needle guide 3 and the movable blade 4 or between the stationary blade 1 and the movable blade 4. The larger diameter of the thicker threads 20 raises the movable blade 4 relative to the stationary blade 1. As a result, the cutting edges 2, 8 of the two blades 1, 4 are unable to cross each other in a suitable fashion, resulting in failure in cutting the thicker threads 20. (See FIG. 4B).